In part because the planks of wood I have are just 140cm long and partly a desire to test out the idea the the width has a more important effect than the length when it comes light wind performance I've gone for a 137x44cm design. Here's the BoardOff Plans.
|BoardOff Plans incl. rocker jigs|
I made the mid section much longer this time at about 400mm and let it taper just 1% of the width between the middle and the end of the mid-section. The straight rails will help up wind in the light conditions. Tips are 375mm wide (355mm+20mm) rails.
I've tweaked the concave/rocker a bit this time. I'm planning on making it from 6mm wood planks. When you run the numbers this roughly doubles the maximum stress in the outer layer of glass. So to improve things I changed the rocker and concave in two ways:
i) Included 6mm of concave at the middle but have it reducing much more rapidly so that its relatively flat just after the footpads. This should stiffen the middle but let the tips flex a bit as I am not going to thin them out.
ii) Used the NHP style rocker line that is curved through the mid-section and relative straight to the ends. However, because I'm building it thinner this time and its likely to flatten in middle when water pressure is on it, I kicked the tips up a bit so that when the mid section is flattened completely the will still be about 10mm of rocker in it to help ride over the chop which plagues us here in DY. This also makes it easier to get the rocker table surface to conform to the jigs as it was a bit easier to avoid introducing kinks.
Overall the board has 30mm of rocker which I'm feeling is a bit too much for really good light wind performance because rocker = drag but the trade of is spray in the which I am thoroughly sick of. This is a bit of an unknown so only time will tell.
In terms of layup, the plan is 2x200gm e-glass top and bottom with 2 x 65mm uni direction carbon tape strips along the length top and bottom which will cover about 30% of the top and bottom of the deck. Uni directional reinforcement is about twice as strong and stiff as woven cloth because of the absence of any crimp in the threads that delays the fibres bearing the load. I've run this through BoardOff's flex model and AEBE the stiffness profile is about 10% less that the same as the 9mm concaveless board from my last attempt. The maximum stresses are still about double, there is no getting away from that as the stress accord to the E-B equation is only a function of the geometry of the cross-section and the distance of the outer layer from the neutral axis of the cross section. It doesn't depend on the elastic modulus of the material. The strain will but the stress won't. So hopefully using the carbon whose elastic module is about 210 GPa compared to e-glass 80GPa will provide the extra strength required to avoid breaking it.
Rails: I bit the bullet and got our local plastics shop to cut the ABS plastic off cut that I had into 10mm strips for using as rails. This time around I'm going to flame treat it properly as last time I used it it was the cause of the delamination that cracked the board under my heal.
The other thing I want to work on is improving the speed of production...